A known X-ray source used for X-ray lithography can be represented by a plasma X-ray source which utilizes the Z pinch established by the discharge of a large current. Owing to the magnetic compression generated in the radial direction, the plasma X-ray source is formed in the shape of a column which is about 1 mm in diameter and 20 to 20 mm in length.
In semiconductor lithography, since fine patterns must be correctly transferred onto the resist, the X-ray that is used must be very close to a parallel beam. In general, therefore, the material to be irradiated is installed on an extension in the axial direction of the column source of light that looks like a point source of light when the light source is viewed from the surface of the resist.
This arrangement, however, has defects as described below when compared with the case where the material to be irradiated is installed on an extension in the radial direction of the column source of light.
In the Z-pinch plasma, a strong compressive magnetic field exists in the radial direction, and high energetic particles in the plasma are confined in the radial direction. However, since the compressive magnetic field does not exist in the axial direction, the high energetic particles are not confined in this direction but are allowed to jet in the axial direction in the form of a shock wave. When the material to be irradiated is installed in the axial direction, therefore, the high energetic particles are introduced together with the X-ray to destroy a thin beryllium disc which constitutes an X-ray pass-through window.
The Z-pinch plasma usually exhibits a large absorption, and the column source of light exhibits a nearly uniform surface intensity. Therefore, depending upon whether the surface of the resist is placed in the axial direction of the column source of light or in the radial direction thereof, the photon flux radiated in the axial direction increases by an area ratio of the source of light as viewed from the surface of the resist.
The above-mentioned problem does not develop if the surface of the resist is placed in the radial direction of the column surface of light. Deviation of the ray of light from the parallel beam that develops when the surface of the resist is placed in the radial direction of the column source of light, can be compensated by placing a band slit as disclosed in Japanese Patent Laid-Open No. 14113/1986. In this case, the following advantages are obtained compared with the case where the surface of the resist is placed in the axial direction of the source of light.
A thin beryllium disc is usually placed between the source of light and the surface of the resist in order to permit the transmission of X-rays only, but to interrupt light of other wavelengths, as well as to maintain a pressure difference between space of discharge and space where the material to be irradiated is placed. The surface of the resist usually has an area of 30 mm square. When the surface of the resist is placed in the axial direction, therefore, a disc having an effective diameter of about 11 mm is obtained if the beryllium disc that is a window material is placed at a position of 1/4 the distance between the source of light and the surface of the resist as measured from the source of light; i.e., the disc having this diameter must be capable of withstanding the pressure difference that includes the shock wave. On the other hand, when the exposure is to be effected in the radial direction of the column source of light via a band slit, the beryllium disc can be placed on the band slit. In this case, the strength of the beryllium disc may be considered relying upon a narrow slit width (about 0.9 mm though it may vary depending upon the conditions). Being compounded by the fact that the shock wave is not transmitted in the radial direction, the thickness of the beryllium plate can be sufficiently reduced when the exposure is to be effected in the radial direction. This offers a great advantage when there is used a source of light of a long wavelength (e.g. a wave length of 12 angstroms in a neon gas environment that can be absorbed well by the beryllium disc.
The photon flux obtained on the surface of the resist is cut off by the band slit and decreases greatly, which, however, is still about 1.5 times as great as the photon flux in the axial direction.